An emergency or normal shutdown of any nuclear power reactor system requires a system to remove decay heat which is produced in the reactor core after shutdown and thereby prevent damage to the reactor and associated systems. The system for removal of decay heat from the reactor core must be one which can remain operational for a lengthy period of time. In power generating nuclear reactor systems, such as a CANDU reactor, a steam generator is present in the primary cooling circuit which acts as a heat sink during normal operation. However, if the steam generator becomes unavailable as a heat sink and the reactor is shutdown, another means must be present to dissipate decay heat which continues to be produced in the reactor core.
U.S. Pat. No. 4,689,194 shows one type of decay heat removal system for a gas cooled reactor. Circulating blowers cause a cooling gas (helium) to flow up through the reactor core and a central hot gas line downward through steam generators and decay heat exchangers back to the blowers. If the circulating blowers are not operational, decay heat from the core is removed by a natural convection flow of the cooling gas in the same direction as the flow during normal operation of the reactor. The decay heat exchangers are each connected with an external re-cooling heat exchanger at a geodetically higher location by means of two legs which form a water circulation loop. If the steam generators are no longer available for the removal of heat from the primary cooling path, they are traversed by hot gas which subsequently passes through the decay heat exchanges. This causes a rise in temperature at the inlet of the decay heat exchangers which leads to evaporation taking place in the water circulation loops whereby natural convection flow in these loops is enhanced and a sufficient amount of heat is removed from the primary cooling path through the decay heat exchangers.
U.S. Pat. No. 4,830,815 shows other types of shutdown cooling systems for pressurized boiling water reactors which include a separate shutdown cooling heat exchanger in a cooling pond. Valves are including in the piping to those heat exchangers and additional pumps are used to pump coolant from the reactor to the shutdown cooling heat exchanger and back to the reactor. These types of systems require valves to be opened and pumps activated before they are operational.